HOW TO......CHOOSE AND DESIGN THE RIGHT SUB BOX.

Why would we want to take a day or so of our own time to end up covered in sawdust and get glue all over our fingers,to make something that might or might not sound any good? Why when we can go into any car accessory shop and buy an off the shelf item that we will be able to audition in our own car first and know before we spend any money if it's any good or not,and have it installed in our car and working in about an hour?

We can buy a ready made active box with the amp built in and packed with a wiring kit that will give us everything we need to get us up and running in next to no time,this will make an easily worthwhile improvement,and still give us change from £100.It’s easier today than it’s ever been,a mere ten years ago about all we had to choose from in ready made bass enclosures were Bazooka Tubes,now the selection available to us runs into the hundreds,so the incentive to DIY it gets less and less all the time.

I guess it comes down to individuality,the sense of achievement when something you’ve made and designed yourself has actually turned into something you can see and listen to,or what we can learn for when we need something that isn’t available off the shelf,and the only option is to make it yourself.

WHAT DIFFERENCE DOES THE SIZE MAKE?

It makes a big difference,the smaller and therefore lighter the cone is,the quicker it can start and stop moving and change direction,meaning the bass will be “tighter” and more accurate,the bigger the cone size the deeper the bass it will reproduce properly as the heavier cone gives it a much lower resonant frequency.

·         8in……….Generally regarded as being the smallest to be classed as a true sub-bass drive unit,very tight and accurate but lacking very deep bass.

·         10in……..About the smallest size that will work well in a medium sized car,reasonably deep and still nice and tight.

·         12in……..The most popular size as they give good deep bass,can still be made to sound punchy,and the box size is still manageable leaving plenty of boot space.

·         15in……..The biggest practical size that is generally regarded as still being “musical”,gives the very deepest bass possible,the box size is now very large,also needs a lot of midbass to keep up with it.

With good choice of driver,careful box design and construction and a good amplifier,the smaller sizes can be made to drop almost as low as the bigger ones.Also the bigger ones can be made to respond very accurately using the same care and attention to detail,but neither will ever be quite as good as the other.

WHY ARE THERE SO MANY DIFFERENT TYPES OF BOXES,AND WHICH ONE IS RIGHT FOR ME?

It all comes down to a lot of different factors,mainly the type of music we listen to,how we listen to our music,and whether we want quality (SQ) or quantity (SPL).The different types of boxes shown below are the most common ones in use in cars,but there are a lot of variations on these basic designs that are beyond the scope of this guide.I’m just covering the simpler designs which are the easiest to get right,the more complicated they get then the easier they are to get wrong,one of the most common things I see in car hi-fi is making things way too complicated just for the sake of it,just remember in this game,less is more!

The 2^nd order sealed  box is still the most popular for a number of reasons,the main being it is easier to build needing only six pieces of wood.The term “2^nd order” means it rolls off its response at the bottom end with a 2^nd order or 12db/octave slope,which is still fairly gradual.

ADVANTAGES

·         Easy to build and not all that critical of volume,get it wrong by as much as 20% and you probably won’t hear the difference.

·         Smaller in volume compared to the same driver in a ported box,by as much as a half.

·         Better power handling when playing deep bass.

·         More gradual frequency roll off at the bottom end,meaning we don’t hear a sudden loss of bass as a note drops suddenly.

·         Sounds a lot more “musical” as the movement of the cone is better controlled by the fixed volume of air trapped in the box.

·         The internal volume of the box can be varied to tune it to give a different sound,better control or better power handling.

DISADVANTAGES

·         The response at the bottom end starts to roll off sooner than the equivalent ported box,so it won’t play the very low notes as well.

·         Inefficient compared to a ported box,meaning it needs more power to get the same output.

Only marginally more difficult to make than the sealed box,but more critical in all aspects of its design and construction.This type of box will play deeper bass,but rolls off a lot quicker than a sealed box,with a 4^th order or 24db/octave slope,which is very noticeable.

ADVANTAGES

·         Plays considerably deeper than a sealed box

·         Is more efficient than sealed,making better use of available amplifier power.

DISADVANTAGES

·         Up to twice the volume of sealed boxes.

·         Very poor power handling below the ports tuning frequency,which can lead to expensive damage to the driver.This can be improved upon by using a very steep slope subsonic filter,but this in turn leads to very low bass almost sounding as if it’s been switched off.A good sealed box can actually play lower if given some help with a graphic or narrow band parametric equalizer.

·         Doesn’t sound quite as musical or controlled as a sealed box.

·         More critical to make than a sealed box,the internal volume as well as the area and length of the port have to be spot on.otherwise performance suffers.

All characteristics are the same as a normal single driver sealed box,but it only needs half the internal volume.The 3db that you would gain from the second driver is lost in this configuration,but good if space is difficult to find.The driver mounted with its magnet outwards must have its phase reversed (so +ve from amp to –ve of speaker and –ve from amp to +ve of speaker) to keep it in phase with the one that is mounted in the box.

ADVANTAGES

·         Very compact box that is half the normal size.

DISADVANTAGES

·         The output level (SPL) of a twin driver configuration is reduced to that of a single driver but it still needs twice the power,making it very inefficient.

·         To work properly the two drivers really do need to be perfectly matched,two apparently identical ones even with consecutive serial numbers won’t always be like this,the mismatch will result in reducing the efficiency even further.

Often called a fifth order enclosure,but not strictly true by itself without an external crossover.Even more difficult to design and get right than any of the previous designs,but in theory gives the best characteristics of both sealed and ported boxes without the disadvantages of either.This type of box forms a mechanical bandpass filter,the rear sealed chamber rolls off the bottom end and the front vented chamber rolls off the top end,so in theory we can put full range sound into it and only get sub bass out,but in practice there is still quite a lot of midrange audible through the walls of the box,so we still need to use the normal type of LPF (Low Pass Filter) to send sub bass only to the box.
This box is a lot more “tuneable” than any of the previous designs,we can adjust the pass band that the box outputs by varying the volume of the front and rear chambers and length of the port.If we tune for a low pass band (e.g. 30-80hz) we will get very good low bass with no loss of power handling etc,but the SPL that the box outputs will be low.If we tune it for a higher pass band (e.g. 40-90hz) we will lose the deep bass but gain on the SPL,making it more efficient.This is all dependant of course on the driver we’re using being suitable for all these different configurations,some would only work with the lower tuning,some with the higher one,or some just won’t work in a band pass box at all.
As a rule of thumb when building boxes like this,the front chamber is half the volume of the rear one,taking into account the volume of the driver and the port of course.

ADVANTAGES

·         The cone movement is better controlled as the volume of air on both sides is constant.

·         There is more potential to tune the box for exactly the type of sound we want than with other types.

·         Plays the deep bass of vented boxes and the tight controlled bass of sealed boxes,but not QUITE as well either one does.

DISADVANTAGES

·         Very difficult to build as ALL the dimensions are extremely critical,get any of them even slightly wrong and the box won’t work properly.

·         The box effectively masks any distortion coming from the subwoofer,meaning you won’t know it’s being overdriven until you smell burning.

·         Bigger than a sealed box but about the same size or slightly smaller than a ported box.

·         Less efficient than a ported box as the entire output is from the port only.

·         As with Isobaric,the two drivers really need to be perfectly matched for this design to work properly,two APPARANTLY identical drivers won’t always be this way.

All the characteristics of the previous bandpass design,but with 3db more output due to the extra driver.In this design all three chambers are the same volume,again taking into account the volume of the drivers and the much bigger port(s) needed for this configuration.
There are a number of other variations of bandpass boxes such as series tuned,in which the rear chamber is connected by a port tube into the front chamber,or in which all chambers are ported to the outside world,each of which are even more critical than any of the previous designs,only to be attempted by the brave or someone with shares in a timber store.

ADVANTAGES

·        More efficient than dual chamber/single driver boxes.

DISADVANTAGES

·        Very large,at least as big as two ported boxes

·        Needs twice the power to drive it.

Strictly speaking not really a box,as it doesn’t rely on the air trapped inside to damp it’s movement,a driver designed for IB or free air use has a much stiffer suspension of its own than ones that are designed to be used in a box with a specified volume.This type of installation rolls off the bass a lot earlier than any of the boxed designs,but much more gradually at only 6db/octave.
The subwoofer is fixed to a wooden baffle board that can either fire through the back seat of a saloon car or coupe,or to a sturdy acoustic shelf in a hatchback car.

ADVANTAGES

·        The frequency response rolls of a lot more naturally meaning no sudden loss of deep bass as with boxed systems.

·        A very small and compact system that takes up very little space.

DISADVANTAGES

·        The power handling will never be as good as with a boxed system.

·        The work involved in installing an IB system can be more than building a box.The seal between the front and back of the subwoofer has to be 100% perfect,even the smallest amount of air that leaks from the back to the front will cause an “acoustic short circuit” that will cancel out a surprising amount of bass.

·        Performance (deep bass) will suffer if we use the boot for its intended purpose of carrying luggage.

·        The number of IB or freeair specific drivers is very limited compared to those intended for boxed use.

A somewhat unusual configuration favoured more by sound off competitors or by those after the best sound quality available in a car.The membrane consists of a very tightly compressed layer of acoustic wadding held between two pieces of wire mesh,and is fitted to the back of the smallest possible box the subwoofer will fit in.It is also possible to screw the sub to the back of it and use it without the box,either way has the same effect.
This has the effect of damping the movement of the cone better than just air in a box ever can,giving the flattest most accurate bass possible,and also a very much easier load for the amplifier to drive.

ADVANTAGES

·        The most accurate bass reproduction possible.

·        A very easy load for the amplifier to drive,vented boxes have VERY wild impedance curves.

·        Your mates will think you’re really cool when you tell them you’ve got one.

DISADVANTAGES

·        As above for the IB system,but with better power handling.

·        Your mates will think you’re a geek when you tell them you’ve got one.

I left this one until last as it isn’t really a properly recognized type of box.As the title suggests it is designed purely to generate as high an SPL as possible,but won’t sound particularly musical.The original designer claimed up to 10db gain over just putting them in normal sealed boxes (equivalent to putting ten times the power into them if they would handle it),but in practice the results are somewhat unpredictable.You may only get as little as 3db gain,or if you’re lucky as much as 6 or 7,but any has to be good if db drag racing is your thing.
There really aren’t any formula for this type of box except the normal rules for downsizing the box to its minimum size for the driver,the size and placing of the baffles is reasonably critical,but would benefit from some experimentation.

ADVANTAGES

·        Generates very high sound pressure level from a very small enclosure.

·        Has excellent power handling.

DISADVANTAGES

·        Somewhat unpredictable as to how well it will work,and also as to which drivers will work in it.

·        Inefficient as any SPL enclosure is,needs a lot of power to make it do anything.

DOES IT MATTER WHAT SHAPE THE PORT IS?

No the actual shape doesn’t matter,but the maths that you do to calculate the volume required for the box will also give the required area for the port.The normal shape is round as any length of cardboard tube or drainpipe can be used,but as long as the area is right than it can be square,triangular,or now the more popular letterbox or slot port.With boxes that will handle a lot of power,it is not a good idea to choose too small a port area,as you will get port noise from the air moving very quickly through it,you will have to choose a larger area which will also mean it will get longer,or a larger number of the same sized ports can also be used.

The slot port is also easier if the port is longer than the boxes largest internal dimension,as shown above,and also less resonant than cardboard tube or drainpipe.

The port doesn’t actually need to be fully inside the box,it can be partly or completely outside of it,meaning in a saloon car the port can exit through the rear shelf,while the box sits on the boot floor,or up against the  bulkhead,and don’t forget to compensate for the difference in internal volume if the port is outside the box.

SO I KNOW WHAT TYPE OF BOX I NEED,BUT HOW BIG SHOULD I MAKE IT?

The easiest way is to use the speaker manufacturers recommended box sizes.This information will normally be supplied on a data sheet that will be packed with it,if not it is normally available on their website as shown HERE.
This gives a number of options for three different sizes of ported boxes,that range from the smallest that will have a higher F3 point (where the response is 3db below the average) through the optimum size to a high output box designed for non-serious SPL use,and also a smaller sealed box designed for sound quality.
Please note that in this case the internal volumes they recommend are the total including the volume of the driver and port(s),whereas other manufacturers would give the amount of air you need left when everything has been put in the box,so in this case you need to add on the volume of everything that goes inside the box,this means the drive unit,the port(s) and any bracing.If they give 1 cu.ft as the optimum volume and everything else is 0.2 cu.ft.then we need to build a 1.2 cu.ft box.Make sure you know which one of these you are using as it can make an audible difference with a ported box.

Sealed boxes are easier to design for the type of sound we want,power handling,deep bass or accurate bass.

SMALLEST (e.g. 0.5 cu.ft for driver X)

·        Best power handling.

·        Highest F3 (no deep bass but peaks well for max.SPL).

·        Poor transient response (Not very tight or well controlled as cone is overdamped).

·        Qtc of 1 or above.

OPTIMUM (e.g.1 cu.ft for driver X)

·        Average power handling.

·        Lowest F3 (best for deep bass).

·        Better transient response.

·        Qtc of 0.7

LARGEST (e.g. 2.5 cu.ft for driver X)

·        Worst power handling.

·        Higher F3.

·        Best possible transient response (very accurate as cone damping is minimal).

·        Qtc of 0.5

WHAT IF I DON’T HAVE THIS INFORMATION,OR JUST WANT TO EXPERIMENT?

The same data sheet that came with it,the website or an e-mail to their technical department will give you some information called the Thiele-Small parameters.These are the results of a number of very involved tests they do that have an almost infinite number of combinations that dictate how it will behave,the most important ones are detailed below:-

·        VAS      (Volume Acoustic Suspension) how stiff the drivers suspension is compared to an equivalent volume of air,measured in cu.ft or litres.

·        QTS      (Q Total of speaker) is the width of the drivers resonant peak,generally lower is better (less than 0.5) as it can give smaller enclosures,but the F3 increases.

·        Fs         The resonant frequency of the driver in free air,which means the frequency it naturally moves easiest at,the lower the better.

·        Re         The DC resistance of the voice coil in ohms (lower than the impedance).

·        Xmax    (Excursion maximum) the one way linear excursion of the driver before it starts to distort or cause damage.

·        Vd         (Volume of driver) the amount of air the driver occupies inside the box.

·        Pmax     (Power maximum) The maximum thermal power handling in ANY enclosure.

Once we have decided which type of box best suits our needs,we need to do something with our Thiele/Small parameters,sealed boxes are easy and can be worked out using simple four function calculator,but vented are a lot more difficult and involve algebra.
There is an easier solution available,and that is by using one of the many bits of freeware/shareware box designer software available,I always use AJ DESIGNS boxbuilder software,not only will it tell you what size box you need,it will also give you a graph of the predicted response.

The term “Qtc” they invite you to enter in the first box is the total Q you need to end up with for a sealed box as detailed above,0.7 being the optimum.

This will provide you with a volume of air needed inside the box,provided the T/S parameters you enter are correct,but why wouldn’t they be correct? In a lot of cases the manufacturer would have measured the T/S parameters on a prototype or pre-production version of the driver,and by the time it goes into production if even one of these changes by even a small amount,it can cause all the others to change as well,so any box we design based on these could be totally wrong.It only needs the cone thickness to change slightly,or even using a different type of glue to cause significant changes.
The only way we can guarantee that a box designed the scientific way will work is to measure the T/S parameters ourselves after properly running the driver in,this means a lot of very involved work with a signal generator,an oscilloscope and some plasticine,there is however a more meaningful way of doing it.The other downside to doing it the technical way is that all it can give us is a two dimensional plot of how that particular driver works in that particular enclosure when fed with a sine wave.Because all music is a series of almost infinitely variable stops and starts,none of this type of software can ever tell us how it will actually sound to our ears playing our choice of music.

THE TRIAL AND ERROR METHOD

I’ve always doubted whether even the most reputable manufacturers of ready made bass systems would ever design them by just sitting down at a computer.I’ve heard so many “scientifically” designed ones that sound awful,and ones where whatever speaker was within reach was dropped in any handy box that have sounded awesome,which all goes to prove that the enclosure makes the biggest difference of all,there is more gain to be found or lost in the box than in any other part of the system.

I see so many cases of small hatchback cars where the owner has crammed two 15in subwoofers in,but has had to downsize the boxes to do this,not realizing that they will lose more then the 3db they will gain from having two drivers instead of one (every time we double the cone area we only get a 3db increase).What this means is that one driver in the right box will have more output than two of them in the wrong boxes,and also the average small car won’t have a big enough alternator to power two of them properly.

We can’t make a small box bigger to find the optimum size,we can however make a large box smaller.The most important first job before we do this is to run the driver in as it will sound totally different when it is brand new and has a very stiff suspension than when it has had some use and has loosened up.Most will need a good 10-15 hours gentle use before they have loosened up sufficiently,some cheap ones will need less,and some very expensive ones could need up to 100 hours.This can be done in-car with music which is easier to hear when it is ready,or on the bench with a 50Hz test tone.

Make yourself the largest box within reason you will need (it doesn’t need to be covered,rough and ready will do as long as it’s airtight),for something like a 12in we wouldn’t need to go any bigger than about 3 cubic feet,then audition it with a piece of music you know well that combines deep and well controlled bass (or whatever takes your fancy),in your car with the rest of the system in place and set up as it will be finally.

Cut yourself some blocks of polystyrene foam (wood or similar will also do) into equally sized pieces of about 0.2 cu.ft. each,and put these inside the box to reduce the internal volume of the box in stages until we get the sound we are looking for.You can also try a port in the same way starting as long as can fit in the car (remember it can be outside of the box),and cutting it down an inch or so at a time.

When you have the sound that is right for you,calculate how much air you now have left in the box,and if you went for ported version,then you also need to calculate the volume the port will take up.Add the port volume onto the volume left inside the test box,and finally build yourself a box to this total volume, THIS gives an idea of how to go about this if you’ve never made one before.

AND FINALLY……

Hopefully by now you will have something close to the sound you’ve been after for all these years.If it isn’t then there are still a number of other factors that affect what the bass sounds like.

·        MIDBASS……..You need good quality amplified midbass or midrange to make the sub bass sound good,listen to just the sub by itself and you’ll hear what I mean,OEM factory fitted speakers just aren’t good enough.

·        BOX INTEGRITY……..The box has to be as solid and well made as possible,this means all the joints must be 100% airtight,made from at least 18mm thick MDF or plywood.and braced where needed,and maybe internal baffles as well.

·        THE SHAPE OF THE BOX……Any shape apart from a cube is good,the less symmetrical the better,most will have at least one sloping face,or according to legend the “golden ratio” is supposed to hold magic powers,which is a box of 1.6 X 1 X 0.6 (high X wide X deep).The only really important thing to remember when working out what dimensions to make the box is to allow at least half the diameter of the sub woofer in between the magnet and the other side of the box,so if you're using a 12in driver,there has to be at least 6in between the magnet and the back of the box.

·        AMPLIFIERS…………Sealed boxes in particular will benefit from having a good quality amplifier driving them,monoblock will always be better than a bridged two channel amp both in terms of quality and efficiency for any type of box.

·        WIRING…………Power cable to the amps must be of sufficient thickness,the earth should be bolted (NOT a self tapper) through a solid bare metal part of the chassis,the lowest loss fuseholder possible such as the ANL type that bolts the fuse link in,all the power connections must be as tight and secure as possible,and nice thick speaker cable (how much oxygen it’s got in the copper really it isn’t all that important,as long as it can carry the current a power hungry sub woofer needs).

·        POWER SUPPLY……..The standard 90 amp alternator can only supply enough current to let your amps produce about 700 watts RMS,as no matter how big or expensive they are they will always be limited by the power supply.You will need to uprate the alternator if you want your kilowatt and above amps to get anywhere there full potential,changing the battery to a heavier duty one or even putting a second one in the boot will do absolutely nothing when the engine is running,as EVERYTHING inside the car will be powered by the alternator and not the battery(s),and also power caps are almost useless if you can’t keep them charged up properly,if you find your car is really struggling to provide enough power for your stereo system,then power caps could make it worse.

Hope you find this helpful and it wasn’t TOO technical,Chris.

Chris another greatttttttt how to and some very descriptive information there.. defo needs to be a sticky

Sat  

Cris

This has to be one of the most informative threads on sound systems i have ever looked at

MOD´S MAKE THIS A STICKY PLEASE

Andy 

Imagewerx I dont have the luxury of being able to put a bass box in my boot, shopping, kids activities etc etc! 

What are your thought on replacing my parcel shelf with an mdf one and fitting either a 10 or 12 inch sub directly into it?

Thanks guys,working out what to do next,any suggestions? I was thinking about "How to write how to's"

afro gti:

Imagewerx I dont have the luxury of being able to put a bass box in my boot, shopping, kids activities etc etc! 

What are your thought on replacing my parcel shelf with an mdf one and fitting either a 10 or 12 inch sub directly into it?

 

No reason why not if you read what I've written about Infinite Baffle installations.

Chris.

Imagewerx:

No reason why not if you read what I've written about Infinite Baffle installations.

Chris.

Doh!... Cheers for the info mate i just re-read your post and saw it staring me in the face.

Pukka post by the way mate!

Imagewerx:

Thanks guys,working out what to do next,any suggestions? I was thinking about "How to write how to's"

afro gti:

Imagewerx I dont have the luxury of being able to put a bass box in my boot, shopping, kids activities etc etc! 

What are your thought on replacing my parcel shelf with an mdf one and fitting either a 10 or 12 inch sub directly into it?

 

No reason why not if you read what I've written about Infinite Baffle installations.

Chris.

how about a how too when buying audio equipment and wiring

why has this not been made sticky ?????   lol

Andy 

Dent Reform:

how about a how too when buying audio equipment and wiring

why has this not been made sticky ?????   lol

I want to do a decent installation one,but don't have any original photos to support it,so if anyone not too distant to me wants to have some gear installed and doesn't mind paying my travelling expenses and buying me a pizza for lunch,then you'll get the job done for free and we can take some photos at the same time?

The how to choose other parts of the system is a good idea,certainly speakers and definately amplifiers,and especially covering how to install them as well.

Chris.

I would like to ask you something more regarding the Isobaric enclosure size.

is there a rough thumb rule to determine what will be the approximate size of isobaric enclosure for either a pair of 12" 8 ohms or 8" 4 ohms woofer for using in a small hatchback car? The mentioned woofers are  Indian made  but there is not much technical specification from the manufacturer accept for the impedance and watts.

I am from a remote mountain called Nagaland where ordering or buying reputed international brand subwoofer is not possible. Also there is no help nearby.

Awaiting your suggestions please.

Regards

Chub2

chub2:

 is there a rough thumb rule to determine what will be the approximate size of isobaric enclosure for either a pair of 12" 8 ohms or 8" 4 ohms woofer for using in a small hatchback car? The mentioned woofers are  Indian made  but there is not much technical specification from the manufacturer accept for the impedance and watts.

If you have did the Thiele-Small parameters then you could work it out using the software I mention above,isobaric would be half the size of the optimum box for any given single driver,but you should be able to get that down to well under 1 cubic foot.
If not you'll have to do it by trial and error after running them in,build a box bigger than you know you'll actually need and then reduce the size gradually in stages by packing it put with sandbags or similar until it sounds how you want it to,then build your proper box the volume of the first box minus the volume of the sandbags.

Chris.

 I don't have the Thiele-Small parameters that's why I am asking your views. regarding a rough approximate enclosure   size for 12" and 8" woofers. By knowing the approximate standard size, one can experiment further.  I don't have any other signal measuring instruments accept for a simple DMM.

Here the speaker manufacturer do not provide the  Thiele-Small parameters or other details that's the problem.

Regards

Chuba 

It is possible to measure the T/S parameters,but you really need a signal generator and an oscilloscope as well as an accurate volt meter,so I think trial and error would be easier.

8in woofers will normally work in between as little as 0.3 cu.ft up to about 0.6 cu.ft,and 12in woofers from about 1 cu.ft up to about 2 cu.ft,all for a single driver of course in a sealed box.

Chris.

 You are right,  with a  signal generator and an oscilloscope one could do the measurement but I can't afford these and there is non in our remote town. That's why I am asking your help hoping there will be another way to do it roughly.

Thanks

Chuba 

As I said,start big and make the boxes smaller until it sounds right,you can make big boxes smaller,but you can't make small boxes larger.

Chris.

P.S. Don't forget to run it in properly before you start all this.

hello i was wondering if you could help? i've made this sub box its got 3 12" subs on this image you can only see two subs but theres is one on the other side, what do you think of the shape? theres no baffles inside. its completely sealed.

cheapcheap:

hello i was wondering if you could help? i've made this sub box its got 3 12" subs on this image you can only see two subs but theres is one on the other side, what do you think of the shape? theres no baffles inside. its completely sealed.

The shape looks fine to me,the more irregular the better,what subs are they,is it just one big cavity inside or split into three seperate ones?

Chris.

 It is just one big cavity inside, do you think it would be beter to spit them off?

The Subs are just some cheap 12" Sub Zero SPLX12RNP 500 watt, £14.99 for testing.

 It's swings and roundabouts whether to have them all share the same airspace or not,it's more efficient if you do (in theory if all the subs are absolutely identical,which cheap ones like these almost certainly won't be),so it COULD be beneficial to give each it's own box.In use though the difference between the individual drivers will sort of average out,and you're probably still better off the way you've got it now.
And don't forget sealed means 100% sealed,99% won't do as ANY air at all that can escape can cause a large amount of cancellation.

Chris.

 well i can get the box home in a few day so i can start testing, Were would you say to put a subwoofer in a room? As i dont yet have a car so they should do in my room powering them with a Cambridge A1 amplifier. should be fun.

Speakers that are designed for in-car use won't sound right in such a large air space,they are designed to use the gain given by the small enclosed space of a car to give a reasonably flat response,the same way but in reverse that normally if you put domestic speakers in a car they will sound very boomy.
They'd have to be tucked into a corner,or even better into an alcove if you have one,my JBL 5.1 sub is in such a place and sounds awesome for a single 8in driver with only 70 watts up it,I only need to move it forward by a couple of feet and it sounds rubbish.

Chris.

 Do you know any decent site's selling speakers for in my home? Also i was wondering about resistance, my my 3 subs are rated a 4Ohms i have 3 what then would the resistance be 1.5, 4, 12 Ohm? As my amp is a   6-16 Ohm amp, if the Ohms are not right can this damage the amp it's self or either the speakers?

Lots of questions ^

Thanks,

Will

cheapcheap:

 Do you know any decent site's selling speakers for in my home?

 Errrmmm..... yes the internet? Or maybe http://www.maplin.co.uk if you want cheap.

Chris.

 i looked on maplins they have some decent seeming speakers. ive just edit the last question!

 My brain hurts

 You can get 1.3333 or 12 ohms from 4 x 4 ohm subs in parallel or series,so go for the series thing and you should find you're in sonic nirvana.

Chris.

woah, thats a lot of info for building a box,

 I found that very interesting read :)
Very well informative
well done!